Both phones are also expected to arrive with the latest version of Google's Android software, 7.1 Nougat.

The announcement will take place during a live event in San Francisco, where the firm is also expected to give more information on its smart home device, Google Home, which was announced in May.

It enables users to talk to Google's artificial intelligence system to find out news, weather and travel information, as well as control smart appliances in their home, including lights and thermostats, using their voice.

The smart speaker is seen as a competitor to Amazon's Echo system which has just gone on sale in the UK.

An updated version of the Google Chromecast, which enables users to stream content from their mobile devices to their TV, is also rumoured to be among Google's plans for the event, with the ability to support 4K definition TV expected to be revealed.

There have also been suggestions of a new version of Android, designed for laptops and tablets, incorporating aspects of the Chrome OS software Google uses for its desktop computers.

Android executive Hiroshi Lockheimer hinted at the change in a tweet last week: "We announced the 1st version of Android 8 years ago today. I have a feeling 8 years from now we'll be talking about Oct 4, 2016."